Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as ultraviolet light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective (sometimes also referred to as “the lens”) and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply a scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection objective, for which usually |β|<1 holds, for example |β|=¼.
An important goal in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
The minimum size of the features that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm and 157 nm and thus lie in the deep (DUV) or vacuum (VUV) ultraviolet spectral range. The next generation of commercially available apparatus will use projection light having an even shorter wavelength of about 13.5 nm which is in the extreme ultraviolet (EUV) spectral range. However, EUV apparatus are very expensive, and thus there is a desire to push the existing DUV and VUV technology to its limits.
One approach in doing so is the use of double patterning exposure technology (DPT). According to this technology, which is particularly useful for layers having a very high pattern density, a single layer is sequentially subjected to two separate exposure and etching steps. For example, a pattern of parallel lines may be lithographically defined and transferred to the layer by etching. This step is repeated, but with the line pattern being laterally displaced. Since the two line patterns interleave, the final line density in the layer is twice the density of the original line pattern. However, the use of this technology is particularly sensitive to overlay errors, because such errors directly translate into undesired line width variations. Since DPT is likely to be used more extensively in the future, the overlay error budget is expected to become significantly smaller.
The term overlay error originally related to the registration of adjacent patterned layers in the microstructured devices. If features that should be arranged one above the other are laterally displaced, this offset is referred to as overlay error. Meanwhile, the term overlay error has also become used to denote relative displacements of features within a single layer.
A more comprehensive understanding of overlay errors involves examining why and to which extent the images of the individual features are laterally displaced. In the case of DPT, no overlay error should be observed if the displacements of feature images that have been defined with different exposures are completely equal. Generally, however, the displacements are different at least to some extent, and thus overlay errors are the rule and not the exception.
For denoting the displacement of an individual feature image the term image placement error (IPE) is frequently used. The image placement error refers to the absolute displacement of a feature image in a layer, i.e. the deviation of the actual image placement from the ideal (desired) placement.
Various causes for image placement errors are known. Among them are alignment errors occurring in the mask and wafer stage. But also the projection objective of the apparatus contributes significantly to image placement errors. One commonly known image placement error is distortion. This aberration denotes an image placement error that depends on the field position and also feature orientation, but that is independent of the feature size and pitch. Distortion is the result of a tilt in the wavefront, which is associated with a particular field point, and is mathematically described by Zernike polynomials Z2 and Z3. There are several approaches to reduce the distortion of a projection objective, among them tilting and/or rotating the wafer and/or the mask, as it is described in US 2004/0263810 A1.
However, often there are also considerable contributions to image placement errors that are a result of other aberrations in the projection objective. It is known that higher asymmetric aberrations which are described by odd Zemike polynomials, for example by Z7 or Z8 (coma), may lead to significant image placement errors. These asymmetric aberrations may be a result of lens heating effects that asymmetrically change the optical properties of the lenses. In contrast to distortion, these contributions depend strongly not only on the orientation of the features, but also on their size and pitch and on the illumination setting. These parameters determine which light directions contribute to the image formation in the image plane of the projection objective, and thus which portions of the exit pupil of the projection objective are illuminated during an exposure. A more detailed discussion of these contributions can be found in an essay by E. Hendrickx et al. entitled “Image placement error: closing the gap between overlay and imaging”, J. Microlith., Microsyst. 4(3), 033006, July-September 2005. This article discusses how image placement errors can be determined either computationally by applying suitable models, or metrologically using SEM measurements, for example.
Another cause for image placement errors is described the article J. Ruoff et al. entitled “Orientation Zernike polynomials: a useful way to describe the polarization effects of optical imaging systems”, J. Micro/Nanolith. MEMS MOEMS 8(3), 031404 (July-September 2009). This article proposes the use of orientation Zernike polynomials (OZP) to describe the polarization properties of the pupil. It is predicted that odd OZP cause image placement errors.
So far, the typical approach for reducing image placement errors depending on feature pitch involves reducing the aberrations that cause the image placement errors. However, this is only feasible to certain extents.